rear discs

I installed rear discs on my factory stock and can't seem to get the pedal to firm up. I replaced the original 15/16 master with a 1.031 one and it helped but still have lots of pedal travel and not enough pressure to the rear.. I have the line on the master the way they were stock with the bigger reservoir going to the front and small to the rear. should I reverse them or is there a better way. Rules require stock type mastercylinder and one only.
When I removed the residual valve I did notice that there are 2 pistons in there separated by a spring I was thinking of putting a solid spacer in there in lew of the spring???